A single atom thick, graphene is the youngest allotrope of carbon and in the last decade it has become most researched material in the scientific community because of its excellent optical, mechanical, and electrical properties. Graphene is the hexagonal arrangement of carbon atoms forming a one-atom thick planar sheet of sp2 hybridized (double bonded) carbon atoms arranged in a honeycomb lattice. Graphene is a promising electronic material. It has the potential to significantly impact the semiconductor industry due to its superior electrical, thermal, mechanical, and optical properties while at the same time offering compatibility with existing semiconductor processing techniques. Graphene has shown extraordinary applications, including single molecule detection, ultrafast field effect transistors (FETs), hydrogen visualization-template for transmission electron microscopy (TEM), and tunable spintronic devices. Furthermore, it exhibits high thermal conductivity (25×silicon), high mechanical strength (strongest nanomaterial), high optical transparency (97.7% for monolayer graphene), carrier controlled interband/optical-transition and flexible structure. Electronically, graphene is a semi-metal with zero band-gap owing to the conduction band touching the valence band at two points (K and K′) in the Brillouin zone. Graphene's high density of π-electrons from the sp2 carbon atoms and carrier-confinement in an open crystallographic structure imparts it with the highest mobility measured to date.
To enhance the charge carrier mobility in interfaced graphene, an atomically smooth, chemically inert and electrically insulator substrate platform is critical. Further, typical silicon-based oxide and nitride substrates are rough and dopant-rich with poor electronic and thermal transport characteristics. See, e.g., Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat Nano 5, 722-726, (2010); and Geim, A. K. & Grigorieva, I. V. Van der Waals heterostructures. Nature 499, 419-425, (2013). In contrast, the wide bandgap hexagonal boron nitride (h-BN) with remarkable physical properties and chemical stability has been recently identified as an ideal gate dielectric for graphene and other two-dimensional nanomaterial (2DN) electronics, deep ultra-violet emission, thinnest tunneling junction, and high chemically tolerant film (for protective coatings). See, e.g., Kubota, Y., Watanabe, K., Tsuda, O. & Taniguchi, T. Deep Ultraviolet Light-Emitting Hexagonal Boron Nitride Synthesized at Atmospheric Pressure. Science 317, 932-934, (2007); Britnell, L. et al. Electron tunneling through ultrathin boron nitride crystalline barriers. Nano letters 12, 1707-1710 (2012); Li, L. H., Cervenka, J., Watanabe, K., Taniguchi, T. & Chen, Y. Strong oxidation resistance of atomically thin boron nitride nanosheets. ACS nano 8, 1457-1462 (2014). Within each layer of h-BN, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are held together by lip-lip interactions in AA′ stacks. See, e.g., Blase, X., De Vita, A., Charlier, J. C. & Car, R. Frustration Effects and Microscopic Growth Mechanisms for BN Nanotubes. Physical Review Letters 80, 1666-1669 (1998); and Golberg, D. et al. Boron Nitride Nanotubes and Nanosheets. ACS Nano 4, 2979-2993, (2010).
Several techniques have been employed to synthesize h-BN, including micromechanical cleavage, chemical exfoliation via ultrasonication, atomic layer deposition and chemical vapor deposition (CVD). The CVD process via decomposition reactions of various BN precursors produces large-area h-BN domains on catalytic metal surfaces. See, e.g., Lee, C. et al. Frictional Characteristics of Atomically Thin Sheets. Science 328, 76-80, (2010); Warner, J. H., Rummeli, M. H., Bachmatiuk, A. & Buchner, B. Atomic Resolution Imaging and Topography of Boron Nitride Sheets Produced by Chemical Exfoliation. ACS Nano 4, 1299-1304, (2010); Debbarma, R., Behura, S., Nguyen, P., Sreeprasad, T. S. & Berry, V. Electrical Transport and Network Percolation in Graphene and Boron Nitride Mixed-Platelet Structures. ACS Applied Materials & Interfaces, (2016); Olander, J., Ottosson, L. M., Heszler, P., Carlsson, J. O. & Larsson, K. M. E. Laser-Assisted Atomic Layer Deposition of Boron Nitride Thin Films. Chemical Vapor Deposition 11, 330-337, (2005); Ferguson, J. D., Weimer, A. W. & George, S. M. Atomic layer deposition of boron nitride using sequential exposures of BCl3 and NH3. Thin Solid Films 413, 16-25, (2002); Song, L. et al. Large Scale Growth and Characterization of Atomic Hexagonal Boron Nitride Layers. Nano Letters 10, 3209-3215, (2010); Kim, K. K. et al. Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition. Nano Lett 12, 161-166, (2012); and Ismach, A. et al. Toward the Controlled Synthesis of Hexagonal Boron Nitride Films. ACS Nano 6, 6378-6385, (2012).
Further, the process requires pre-treatment steps such as electrochemical polishing or chemical mechanical polishing and high temperature annealing, respectively. Subsequently, relocating h-BN from metal surfaces to other dielectric substrates requires additional wet/dry transfer process which introduces unintentional surface corrugations and possible adsorption of polymeric impurities on h-BN surface. Therefore, direct, transfer-free and scalable synthesis of h-BN films on dielectric surfaces is critical for 2D electronics and industry-scale applications. See Tay, R. Y. et al. Direct growth of nanocrystalline hexagonal boron nitride films on dielectric substrates. Applied Physics Letters 106, 101901, (2015); and Wang, M. et al. A Platform for Large-Scale Graphene Electronics—CVD Growth of Single-Layer Graphene on CVD-Grown Hexagonal Boron Nitride. Advanced Materials 25, 2746-2752, (2013).
Limited work has been performed on producing amorphous, nano-crystalline and poly-crystalline h-BN films on SiO2/Si surfaces via thermal and plasma-enhanced CVD. See Li, Q., Jie, Y., Mingyu, L., Fei, L. & Xuedong, B. Catalyst-free growth of mono- and few-atomic-layer boron nitride sheets by chemical vapor deposition. Nanotechnology 22, 215602 (2011); Hirayama, M. & Shohno, K. CVD-BN for Boron Diffusion in Si and Its Application to Si Devices. Journal of The Electrochemical Society 122, 1671-1676, doi: 10.1149/1.2134107 (1975); and Rand, M. J. & Roberts, J. F. Preparation and Properties of Thin Film Boron Nitride. Journal of The Electrochemical Society 115, 423-429, doi: 10.1149/1.2411238 (1968).